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CHE202 Reductions and Heterocycles

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CHE202 Reductions and Heterocycles CHE202 Structure & Reactivity in Organic Chemistry: ! Reduction Reactions and Heterocyclic Chemistry! 9 lectures, Semester B 2014! Dr. Chris Jones! ! [email protected]! Office: 1.07 Joseph Priestley Building! ! Office hours:! 9.30-10.30 am Tuesday! 1.30-2.30 pm Thursday (by appointment only)! Course structure and recommended texts! 2! §" Coursework:! !Semester B – week 9 ! !5% (‘Coursework 7’)! !Semester B – week 11! !5% (‘Coursework 8’)! ! §" Test:! !Semester B – week 12! !15% (‘Test 4’)! §" Recommended text books:! ‘Organic Chemistry’, Clayden, ‘Oxidation & Reduction in ‘Heterocyclic Chemistry’, Greeves & Warren, OUP, 2012.! Organic Chemistry’, Donohoe, Joule & Mills, Wiley, 2010.! OUP, 2000.! Don’t forget clickers! Overview of Reduction Chemistry lecture material! 3! §" Reduction:! - Definition (recap.)! - Reduction of carbon-carbon double and triple bonds! - Heterogeneous hydrogenation! - Homogeneous hydrogenation, including stereoselective hydrogenation! - Dissolved metal reductions! - Other methods of reduction! - Reduction of carbon-heteroatom double and triple bonds! - Reduction of carbonyl derivatives, addressing chemoselectivity! - Stereoselective reduction of carbonyl derivatives! - Reduction of imines and nitriles! - Reductive cleavage reactions! - Hydrogenolysis of benzyl and allyl groups! - Dissolved metal reduction! - Deoxygenation reactions! - Reduction of heteroatom functional groups! e.g. azides, nitro groups, N-O bond cleavage! Reduction: definition! 4! §" Reduction of an organic substrate can be defined as:! - The concerted addition of hydrogen.! e.g.! ! O H2 (g) H O H Catalytic hydrogenation (e.g. H2(g) & Pd)! ! - The ionic addition of hydrogen NB: Dr. Lebrasseur’s & Dr Bray’s carbonyl lectures.! e.g.! ! O "H " then H Hydride addition then protonation! H O H (e.g. LiAlH4 then acid w/up)! ! - The addition of electrons.! e.g.! H H ! electron transfer Dissolved alkali metals! H H one electron added! ‘OIL RIG’: a helpful mnemonic...! 5! §" Consider the reaction from the point of view of the electrons:! ! OIL !Oxidation Is Loss! RIG !Reduction Is Gain! Question: reduction or not?! 6! §" Which of these transformations represent reductions? ! A. All of them! 1 B." 3, 4 and 5! C." 1, 3, 4 and 5! D." 1, 3 and 5! E." 1, 4 and 5! NO2 NO2 NO2 NH2 2 3 N N N N O O O N 4 NH2 CO Me CO Me 2 5 2 §" Reduction of carbon-carbon double and triple bonds! Reduction of carbon-carbon double and triple bonds! 8! §" Catalytic hydrogenation! - Concerted addition of hydrogen across a #-bond.! - Use hydrogen gas.! - Transition metal (TM) catalyst promotes the reaction.! - Catalyst can be heterogeneous or homogeneous.! H H Metal Catalyst H H2 (g) H - Hydrogenation has a different mechanism of reduction compared to hydride reducing agents (e.g. NaBH4), therefore different chemoselectivity is often observed.! e.g. ! O O Pd/C (10 mol%) H H H2 (g) Aldehyde not reduced! NB: 10 mol% = 0.1 equivalent! Reduction of carbon-carbon double and triple bonds! 9! § Heterogeneous hydrogenation! - Catalyst insoluble in reaction medium.! - TM (e.g. Pt, Pd, Rh) adsorbed onto a solid support, typically carbon or alumina (Al2O3).! §" Reduction of alkenes:! - Reactions generally proceed at r.t. and 1 atm. H2 pressure, however, reaction rates increase when elevate T and/or P.! - Hydrogenation is typically selective for syn-addition.! e.g.! Ph Ph PtO2 (cat.) (R)! Ph Ph (S)! H H2 (1 atm.) H (Z)-alkene! PtO2 (cat.) (S)! Ph Ph Ph (S)! Ph H H2 (1 atm.) H (E)-alkene! - Need a solvent that dissolves sufficient hydrogen (e.g. methanol, ethanol, acetic acid).! NB: Very occasionally the product from anti-addition occurs, see p. 624 OC textbook for an explanation.! Reduction of carbon-carbon double and triple bonds! 10! §" Mechanism:! - Complex and difficult to study (reaction occurs on metal surface and each catalyst is different).! - Working model (explains syn-selectivity):! i). H2 dissociatively adsorbed onto metal surface.! ii). Alkene #-bonds coordinated to catalyst surface.! iii). Alkene #-bonds adsorbed onto catalyst surface.! iv). A hydrogen atom is added sequentially onto both carbons.! v). Reduced product can dissociate from catalyst surface.! syn-addition! R R R R H2 H H ADDITION ADSORPTION then DISSOCIATION R R R R R R R R H H H H R R H H R R metal catalyst surface COORDINATION ADSORPTION - Syn-selectivity increases with increased hydrogen pressure.! - Reactivity decreases with increased alkene substitution.! Reduction of carbon-carbon double and triple bonds! 11! §" Complete reduction of aromatic compounds:! - Lose aromaticity: more forcing conditions required c.f. isolated alkenes.! - Rh, Ru & Pt are most effective catalysts (i.e. Pd less active so use Pd when require chemoselectivity for alkene reduction in presence of aromatic ring).! - Carbocyclic and heterocyclic aromatic rings amenable.! HO O HO O OH OH Pt/C (cat.), H (4 atm.) Carbocyclic ring! O 2 O NB: carboxylic acid untouched! OH AcOH OH O O NB: increased H2 pressures! H 10% PtO2 (10 mol%) H2 (5 atm.), HBr, r.t. Heterocyclic ring! N NB: syn-addition N of hydrogen! 74% Bun Me n Me Bu (±)-monomorine! Question: reduction of carbon-carbon double and triple bonds! 14! §" Which of these structures 1 to 5 is the correct reduction product?! A. 1! O B." 2! Pd/C (10 mol%), H2 (1 atm.) ? C." 3! MeOH, r.t. D." 4! E." 5! OH O O H H H H H H 1 2 3 O O H H H H 4 5 Reduction of carbon-carbon double and triple bonds! 12! §" Reduction of alkynes to alkanes:! - Standard heterogeneous hydrogenation results in complete reduction to alkanes.! e.g.! H H O Pd/C (10 mol%) Ph OMe Ph 2 moles of H2 added! OMe H2 (1 atm.), MeOH H H O 1 mole of H2 added! Ph CO2Me H H alkene intermediate! - Alkynes to alkenes?! - Require chemoselectivity to differentiate between reduction of alkyne and alkene.! ! - Require control over cis- or trans- geometry. ! Reduction of carbon-carbon double and triple bonds! 13! § Reduction of alkynes to cis-alkenes! - Lindlar’s catalyst: affords cis-alkenes.! - Pd is poisoned with Pb and an amine – more active towards alkyne than alkene (NB: alkene reduction is still possible so reactions often require careful monitoring).! e.g.! Herbert Lindlar! O Pd/CaCO (10 mol%) Ph CO2Me 3 Solid support – CaCO or BaSO Ph 3 4! H2 (1 atm.), quinoline, OMe H H Pb(OAc)4 – deactivates catalyst! Pb(OAc)4, EtOAc Quinoline – competitive cis-(Z)-alkene! binding to catalyst surface! e.g.! O O N Pd/CaCO3 (10 mo%) O O H2 (1 atm.), quinoline TBSO O Pb(OAc)4, hexane, r.t. TBSO R R O 86% - Mechanism: two hydrogens added to same face of alkyne (similar to slide 10), giving syn-addition.! Reduction of carbon-carbon double and triple bonds! 15! §" Homogeneous hydrogenation:! - Metal-ligand complex is soluble in reaction medium.! - Phosphines are common ligands (i.e. good donors).! ! Sir Geoffrey Wilkinson! (Nobel Prize in Chemistry 1973)! e.g. Wilkinson’s catalyst, (Ph3P)3RhCl! - Stereospecific syn-addition of hydrogen across alkene.! - Less substituted and least sterically hindered double bonds reduced most easily.! - Chemoselectivity (i.e. ketones, carboxylic acids, esters, nitriles, ethers and nitro groups all inert to these conditions.! Least hindered alkene! O O (Ph P) RhCl (cat.), H (1 atm.) O 3 3 2 O O benzene/EtOH O 95% NB: Heterogeneous hydrogenation (e.g. H2, Pd/C) is non-selective and leads to over reduction! NB: This mechanism is beyond the scope of the course – for a full explanation see ‘Ox & Red in Org Synth’ p. 54! Reduction of carbon-carbon double and triple bonds! 16! § Enantioselective hydrogenation:! H2 - If metal-ligand complex (MLn) is chiral, then possible to control to which face of alkene H2 is delivered.! - Discrimination between enantiotopic faces of alkene MLn + WHICH FACE? can lead to single enantiomer of product.! H2 ANSWER: use chiral ligand.! e.g. BINAP is a chiral & bidentate ligand for TM.! - Commercially available as both (R)- and (S)- enantiomers (i.e. chose desired product enantiomer).! Bottom face! top face! CO2H CO2H PPhRh(I), (R)-BINAP CO2H Rh(I), (S)-BINAP H 2 H PPh NHCOPh 2H2 (1 atm.) NHCOPh H2 (1 atm.) NHCOPh Very high facial selectivity (98% ee)! NB: this is a protected version (S)-BINAP of the amino acid - alanine! - Beyond hydrogenation, homogeneous catalysis is an extremely important area of organic chemistry and you will encounter numerous examples in future lecture courses…! Reduction of carbon-carbon double and triple bonds! 17! §" Dissolved metal reductions:! - Alkali metals (e.g. Na, Li, K) dissolved in liquid ammonia.! - ‘Free electrons’ add to low-lying #*-orbitals.! NH3 (l) Consider the solution as a Na Na + e source of ‘free electrons’! §" Reduction of alkene to alkane:! - Selectively reduces electron deficient alkenes to alkanes (i.e. no reaction with standard alkenes).! O O i). Li, NH3 (l), t-BuOH (1 equiv), –78 ˚C ii). NH4Cl Reduces electron deficient alkene only! NB: Low temperature as NH3 is a gas at T > –33 ˚C.! NB: Stoichiometry of alcohol is crucial.! Reduction of carbon-carbon double and triple bonds! 18! §" Mechanism of alkene reduction:! i). Addition of electron to #-bond gives radical anion.! ii). Radical anion protonated by 1 equiv. of t-BuOH to give neutral radical.! iii). Addition of 2nd electron yields allylic anion.! iv). Proton transfer affords enolate (NB: no proton source so enolate stable under these conditions).! v). Addition of more acidic proton source, NH4Cl (aq), protonates enolate to give ketone product. ! ! O O OH + e H Ot-Bu Single electron transfer! Single + e electron transfer! O O PROTON OH H TRANSFER No t-BuOH remaining – enolate stable until add NH4Cl (aq)! Reduction of carbon-carbon double and triple bonds! 19! § Reduction of alkynes to trans-alkenes:! - Dissolved metal reduction forms trans-double bond with high levels of stereoselectivity.! e.g.! c.f.
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